a. Field of Invention
This invention pertains to a system for measuring a particular dimension of an object, and more particularly to the non-contact measurement of the differential thickness of a relatively thin object by automated means.
b. Description of the Prior Art
As stronger and lighter materials are being developed, it has become possible to use very thin sheets of such materials in various application where previously much thicker and heavier materials were necessary. For example, cans for dispensing various popular beverages to the general public are made of much thinner sheet metals than in the past. With this development there arose a need for equipment capable of making very small non-contact measurements for quality control, and more particularly for differential thickness measurements. Heretofore such measurements could only be made by making physical contact with the article to be measured. However, this type of testing was labor intensive, slow and inaccurate.
The following references are illustrative of the state of the art:
U.S. Pat. No. 4,498,778 to White discloses a method and device for measuring the spatial coordinates of a workpiece by illuminating the workpiece with a planar light source generating an electronic image of the workpiece and analyzing the image with a computer. Only one surface of the workpiece is illuminated and the features of the image used for analysis correspond to various features of the workpiece itself.
U.S. Pat. No. 3,736,968 to Mason discloses a method and apparatus for positioning logs, in which the dimensions of a log are determined by illuminating the log with two facing light sources 135, 136 and analyzing the light reflected therefrom.
U.S. Pat. No. 3,187,185 to Milnes discloses a device for determining the contour of a sheet of steel. Two beam sources generate two straight lines on the same side of a sheet. The distance between the lines indicates the thickness of the sheet. Two other beam sources are used to generate two more lines on the opposite sheet surface. The configuration of the lines, when superimposed yields information on the sheet contours. However the distance between lines dispersed on opposite sides of the sheet is not indicative of the thickness of the sheet.